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Abstract

Northeast Japan experienced an
approximately constant,
compressional deformation during
the last 5 million years resulting from
the steady subduction of the Pacific
plate. Because the direction of the
maximum compression axis is
approximately perpendicular to the
strike of the island arc, 2-D
finite-element modeling can be used
to examine the deformation over
time of the island-arc lithosphere.
The model geometry is based on
geophysical and geological data, and
each model run requires an assumed
rheology and interplate coupling.
Novel to our modeling is the ability
to include erosion/deposition loading
and the creation of strike-slip faults,
based on a dynamically-applied
fracture criterion. The criterion for
acceptability is how well a model
matches observed present-day
topography, gravity, and seismicity
patterns. Results given below are for
models that satisfy this criterion.

The long-term effective elastic
thickness is 10 km in the inner arc,
increasing to about 50 km near the
trench. The effective elastic
thickness in the inner arc is therefore
much smaller than the about 30 km
short-term elastic thickness
estimated from seismological data.
The viscosity of the lower crust is on
the order of 1022 Pa s or less.

The strength of interplate coupling
off Sanriku is about two to four
times greater than off Miyagi, and
there is about twice as strong a
coupling at greater depths. The
relative strength of coupling
correlates well with the observed
interplate seismicity. Hence the
inferred weaker coupling off Miyagi
indicates a lack of seismogenic
potential -- a low probability for
large earthquakes in that region, not
just a long return cycle.

The same modeling procedure was
also applied to southwest Japan.
The viscosity of the lower crust is
not more than 1021 Pa s, and the
elas tic thickness is about 10 km.
The calculated strength of interplate
coupling for southwest Japan is
about 1.5 times greater than for the
off-Sanriku region in northeast
Japan, which correlates well with the
fact that there have been great
(M>8) earthquakes in the Nankai
Trough region, but none that large in
the off-Sanriku region.